1. Field of the Invention
The present invention relates to a power supply circuit that steps down an input voltage and supplies it to a load. In particular, the present invention relates to a step-down chopper regulator that obtains a drive voltage to be given to a gate of an output power transistor by a bootstrap method.
2. Description of Related Art
Improvement of power conversion efficiency causes effects of energy saving, long life of batteries, reduction of heat and the like, so it is the most important task for a switching power supply. In addition, as a result of recent promotion of energy saving, equipment to be supplied with power from the switching power supply has become to support low voltage, i.e., capable of working at low voltage like 2.5 volts or 1.5 volts in general. On the other hand, current required by the equipment is tending to increase. In the switching power supply, a power loss due to on-resistance of a switching element that increases in proportion to increase of current flowing in the equipment is a main factor that lowers the power conversion efficiency. Therefore, it is important task how to reduce the on-resistance of the switching element.
Note that the on-resistance of the switching element can be reduced by increasing a size of the switching element, but the increase of size of the switching element must be minimized because it causes increase of cost. In addition, comparing an N-channel MOS transistor (or an NPN bipolar transistor) with a P-channel MOS transistor (or a PNP bipolar transistor) as the switching element, the N-channel MOS transistor (or an NPN bipolar transistor) is more preferable because its chip size can be reduced for integration. However, since a bootstrap type gate voltage generating circuit (hereinafter referred to as a bootstrap circuit) is necessary for driving the N-channel MOS transistor, it is necessary to constitute the bootstrap circuit at low cost.
FIG. 21 is a circuit diagram showing an example of a conventional step-down chopper regulator that uses a bootstrap circuit.
The bootstrap circuit shown in FIG. 21 has a structure in which a boot diode 106 and a boot capacitor 107 are connected in parallel with an output power transistor 100 (the switching element) of the step-down chopper regulator. When the output power transistor 100 is turned off, an input voltage Vin charges the boot capacitor 107 through the boot diode 106. Therefore, a boot voltage Vboot that is applied to a drive circuit 102 is higher than an output voltage Vout (that is source voltage of the output power transistor 100) by charged voltage of the boot capacitor 107 (Vin−Vf) (Vf is forward drop voltage of the boot diode 106, which is approximately 0.4 volts).
Furthermore, in a case of a single chip IC that includes the output power transistor 100, the output power transistor 100 is usually made up of a laterally diffused MOS transistor (hereinafter referred to as LDMOS) that has high drain withstand voltage and is capable of reducing the on-resistance per unlit area.
An example of a conventional technique related to the above description is disclosed in JP-A-H5-304768, JP-A-2000-92822 and the like.
JP-A-H5-304768 discloses and proposes a DC-DC converter having a structure in which a MOS-FET is used as the switching element, a high input voltage Vi is converted into a low output voltage Vo, the output voltage Vo is compared with the reference voltage by a pulse width control IC so that open and close of the switching element is controlled via a gate driving circuit. This DC-DC converter includes a constant voltage circuit disposed that stabilizes gate drive voltage for the switching element between the gate driving circuit and an input power source terminal.
In addition, JP-A-2000-92822 discloses and proposes a drive power supply circuit for a semiconductor switching element having a structure in which a plurality of semiconductor switching elements are connected in series between the positive and the negative electrodes of a first DC power source, a first capacitor is connected in parallel with a second DC power source, the anode of a first diode is connected to the positive electrode of the second DC power source, a series circuit of a second diode, a second capacitor and a first transistor, and a series circuit of a zener diode, a resistor and a second transistor are connected in parallel between the cathode of the first diode and the negative electrode of the second DC power source, a third capacitor is connected in parallel with the zener diode, a third diode is connected between the node of the second diode and the second capacitor and the node of the zener diode and the resistor, a third transistor is connected between the node of the second capacitor and the first transistor and the node of the zener diode and the resistor, and the gate terminal of the third transistor is connected to the node of the resistor and the second transistor. The first and the second transistors are driven to be turned on and off alternately by an oscillator circuit, so that voltage of the third capacitor is used as the drive power for a semiconductor switching element on the positive side.
It is surely able to use the N-channel MOS transistor as the output power transistor 100 in the bootstrap type step-down chopper regulator shown in FIG. 21, so that a chip size of the integrated circuit can be reduced compared with the case where the P-channel MOS transistor is used.
However, in order to provide a bootstrap type step-down chopper regulator IC that includes the output power transistor 100 in a single chip for supporting low cost required recently, it is necessary to use the BiCDMOS (Bipolar Complementary Double-diffused MOS) process for making the boot diode 106 that requires the bipolar technique (including epitaxial steps), an LDMOS transistor used as the output power transistor 100 and CMOS (Complementary MOS) transistors that form other circuit portion (a main logic generating circuit 101 and the drive circuit 102 in FIG. 21) in a single wafer. Therefore, it is unnecessary to prepare the output power transistor 100 as a discrete component, but cost of the step-down chopper regulator IC increases. In addition, a much more expensive process is necessary for making the boot diode 106 as a Schottky barrier diode in order to support high speed oscillation.
In addition, as to the conventional bootstrap circuit shown in FIG. 21, the boot voltage Vboot varies in accordance with the input voltage Vin. Therefore, if the input voltage Vin is low, a level of the gate voltage for the output power transistor 100 decreases. On the contrary, if the input voltage Vin is high, the level of the gate voltage for the output power transistor 100 increases. For this reason, in the conventional bootstrap circuit described above, the input voltage Vin should be set by considering a gate withstand voltage of the output power transistor 100. It is unable to set exceeding the above-mentioned gate withstand voltage. In particular, if the output power transistor 100 is made up of the LDMOS transistor, the gate withstand voltage thereof is 10 volts or less in many cases so that the input voltage range becomes narrow.
Note that the conventional technique described in JP-A-H5-304768 proposes to provide the constant voltage circuit that stabilizes the gate drive voltage of the switching element between the gate driving circuit and the input power source terminal so as to supply a constant gate drive voltage regardless of the input voltage. However, the above-mentioned conventional constant voltage circuit generates the constant voltage based on the output voltage (the switched voltage having a rectangular waveform) as a reference voltage, so the constant voltage circuit has a very complicated structure.
In addition, the bootstrap type step-down chopper regulator is preferably required to deliver the output voltage Vout having a rectangular waveform, but it may be discontinuous mode in which the coil current Ic flowing in an output inductor 103 is not continuous if the coil current Ic is little. In this case, the boot capacitor 107 is charged insufficiently so that gate-source voltage of the output power transistor 100 rises insufficiently resulting in malfunction in the switching action.